The present application claims priority to Japanese Patent Application No. 2001-308529 filed in the Japanese Patent Office on Oct. 4, 2001, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a transfer device and an image-forming apparatus including the transfer device, and more particularly to the control of an adsorbing bias applied to a surface of a recording medium so that the recording medium is electrostatically adsorbed to a transfer element.
2. Discussion of the Background
In an image-forming apparatus such as a copying machine, a printer, a facsimile machine, or other similar image-forming apparatus, an electrostatic latent image formed on a photoreceptor functioning as an image carrier is developed with toner to obtain a visual image, i.e., a toner image. Next, the toner image is transferred onto a recording medium such as a recording sheet in a transfer process.
With regard to the transfer process, in the case of single color image formation, a toner image may be directly transferred from a photoreceptor to a recording sheet. In the case of multi-color image formation, toner images of different colors formed on a photoreceptor may be sequentially transferred onto an intermediate transfer element, i.e., a primary transfer, while being superimposed upon each other on the intermediate transfer element. The superimposed multi-color image on the intermediate transfer element may be collectively transferred onto a recording sheet, i.e., a secondary transfer.
An intermediate transfer element may be shaped in the form of drum or a belt. When the intermediate transfer element is shaped in the form of a belt (hereafter referred to as an xe2x80x9cintermediate transfer beltxe2x80x9d), instead of collectively transferring a superimposed multi-color image onto a recording sheet fed from a sheet feeding device, it has been proposed that a recording sheet is adsorbed to the intermediate transfer belt. Further, toner images of different colors formed on respective photoreceptors are sequentially transferred onto the recording sheet, which moves together with the intermediate transfer belt, while being superimposed upon each other on the recording sheet. This technology is described in, for example, Japanese Laid-open Patent Publication Nos. 63-118780, 5-270686, and 8-152790.
The above-described Japanese Laid-open Patent Publication Nos. 63-118780, 5-270686, and 8-152790 employ a construction in which a plurality of photoreceptors arranged along the intermediate transfer belt in the direction of movement of the intermediate transfer belt, and toner images of different colors formed on the respective photoreceptors are sequentially transferred onto one side of a recording sheet adsorbed to the intermediate transfer belt. That is, the construction allows an image to form on only one side of the recording sheet. Another construction is described, for example, in Japanese Laid-open Patent Publication No. 2001-109325, in which toner images of different colors formed on respective photoreceptors are sequentially transferred onto a first (front) side of a recording sheet, and then subsequent toner images of different colors are sequentially transferred onto a second (rear) side of the recording sheet.
An image-forming apparatus in which a plurality of image-forming units (including photoreceptors) are arranged along an intermediate transfer belt as an intermediate transfer element in the direction of movement of the intermediate transfer belt is called a tandem-type image-forming apparatus. A tandem-type image-forming apparatus often uses toner of four colors, including black toner.
In the above-described background transfer constructions, toner images of different colors are sequentially transferred from photoreceptors onto a recording sheet by applying a transfer bias to the recording sheet via an intermediate transfer belt (hereafter referred to as a xe2x80x9ctransfer beltxe2x80x9d) each time the recording sheet passes transfer positions. As a result, the charge of the recording sheet increases by applying a transfer bias to the recording sheet.
For example, in a multi-color image-forming apparatus that has been widely used recently, an electrostatic latent image formed on a negatively charged photoreceptor is developed with negatively charged toner and formed into a toner image. In the transfer process, the toner image is transferred onto a recording sheet by applying a transfer bias having a positive polarity with respect to the recording sheet. In this image-forming process, each time the recording sheet is separated from the photoreceptor when passing transfer positions, electric discharge is generated between the photoreceptor and the recording sheet. As a result, the negative charge of the recording sheet becomes higher each time the recording sheet passes the transfer positions.
When forming images on both sides of a recording sheet, after the transfer process for the first side of the recording sheet is completed, a subsequent transfer process for the second side of the recording sheet is performed. In this case, a bias for adsorbing a recording sheet to a transfer belt (hereafter referred to as an xe2x80x9cadsorbing biasxe2x80x9d) to be applied to the second side of the recording sheet must be changed from that applied to the first side of the recording sheet. The adsorbing bias is changed according to the change of electric resistance of the recording sheet caused by the change of humidity of the recording sheet in the fixing process performed after the transfer process for the first side of the recording sheet. In the fixing process, a toner image is fixed onto the recording sheet under the influence of heat and pressure. Thus, by having to change the adsorbing bias applied to the second side of the recording sheet from that applied to the first side of the recording sheet, adsorbing bias control becomes complicated.
In order to improve the above-described negative charging condition of a recording sheet, a background image-forming apparatus may use a discharging AC charger. For example, Japanese Laid-open Patent Publication No. 7-199679 describes an image-forming apparatus in which, after completion of a transfer process for the first side of a recording sheet, a discharging AC charger discharges the recording sheet. Subsequently, a transfer process for the second side of the recording sheet is performed without changing the adsorbing bias to be applied to the second side of the recording sheet from that applied to the first side of the recording sheet.
However, with increasing demands for environmental protection, such as reduction of ozone production, and for cost reduction of an apparatus, an image-forming apparatus tends to have a construction that lacks a discharging device like the discharging AC charger. In a construction without a discharging device, a negative electric charge given to a recording sheet in a transfer process for the first side of the recording sheet must be cancelled by re-charging the recording sheet, before performing a transfer process for the second side of the recording sheet. In this case, an adsorbing bias having an opposite polarity (i.e., a positive polarity) may be required to be applied to the recording sheet so as to cancel the negative electric charge given to the recording sheet in transfer operations which are repeated four times in the transfer process for the first side of the recording sheet.
When forming images on both sides of a recording sheet, the charging conditions of the first and second sides of the recording sheet may be different, due to the difference of humidity between the first and second sides of the recording sheet. When forming an image on the second side of a recording sheet, since the percentage of moisture content of the recording sheet decreases after the fixing process for the first side of the recording sheet, the second side of the recording sheet tends to have a high resistance compared to the first side of the recording sheet. Therefore, when a transfer operation for each color is repeated for the second side of the recording sheet, a negative electric charge generated in the transfer process remains on the second side of the recording sheet. Thus, the second side of the recording sheet has a considerably high negative charge compared to the first side of the recording sheet. As a result, negatively charged toner transferred onto the second side of the recording sheet is in an electrically unstable condition. This causes toner scattering, in which electrically unstable toner of a toner image on the second side of the recording sheet scatters when the recording sheet is separated from a transfer belt (after completion of the transfer process for the second side of the recording sheet).
Therefore, considering the high resistance and high charging condition of the second side of the recording sheet, a high adsorbing bias, having a polarity opposite to that of the electric charge given to the recording sheet in the transfer process for the first side of the recording sheet, may need to be applied to the recording sheet before performing the transfer process for the second side of the recording sheet. For example, when the electric charge given to the recording sheet in the transfer process for the first side of the recording sheet has a negative polarity, the adsorbing bias applied to the recording sheet before the transfer process for the second side of the recording sheet may need to have a positive polarity to cancel the negative electric charge given to the recording sheet.
Next, consideration will be given to an adsorbing bias applied to a recording sheet when forming an image on the first side of the recording sheet in a dual-side image-forming mode.
When forming an image on the first side of a recording sheet, which depends on environmental conditions that influence the percentage of moisture content of a recording sheet, the recording sheet tends to have a low resistance under the high humidity condition, due to an increase in the moisture content of the recording sheet. With such a low resistance of the recording sheet, when an adsorbing bias having a polarity (e.g., negative) opposite to that of a transfer bias (e.g., positive) is applied to an adsorbing bias applying device, an electric field between the adsorbing bias applying device and a transfer bias applying device for the first color toner image increases. Thus, a positive transfer bias charge flows into the adsorbing bias applying device through the recording sheet. As a result, an inferior transfer of a first color toner image occurs. Specifically, when the recording sheet has a surface resistivity of, for example, 5xc3x971010 xcexa9/square, an image in which a first color is conspicuous is formed. This phenomenon tends to occur when the space between the adsorbing bias applying device and the transfer bias applying device for the first color toner image is reduced for downsizing the apparatus.
On the other hand, when the polarities of the adsorbing bias and the transfer bias are different, and when a recording sheet has a high resistance, the recording sheet is charged with the negative adsorbing bias applied from the adsorbing bias applying device. As transfer operations are repeated at transfer positions, the negative charge of the recording sheet increases, thereby causing an inferior transfer in which a negatively charged toner image is not smoothly transferred from a photoreceptor to the recording sheet (even though the positive transfer bias is applied to the recording sheet). As a result, a deteriorated image tends to be obtained. In addition, for increasing the transfer bias voltage in succeeding transfer operations in order to prevent image transfer efficiency from decreasing, a power supply having a big electric power capacity needs to be provided to increase the transfer bias voltage at transfer positions located downstream in the sheet-moving direction. This may increase the cost of the apparatus. For the above-described reasons, when performing the transfer process for the first side of the recording sheet, the adsorbing bias and the transfer bias preferably have the same polarities.
When an adsorbing bias having a polarity (e.g., positive) equal to that of a transfer bias (e.g., positive) is applied to the adsorbing bias applying device, the electric field between the adsorbing bias applying device and the transfer bias applying device for the first color toner image attenuates. However, when a recording sheet has low resistance, a positive adsorbing bias charge flows to a negatively charged photoreceptor carrying the first color toner image through the recording sheet. As a result, an excess amount of the transfer bias is produced, thereby deteriorating the image. Particularly, when the recording sheet has a surface resistivity of, for example, 5xc3x971010 xcexa9/square, an image in which a toner image is partially omitted is formed. Because, such a problem tends to occur as the adsorbing bias increases, the value of the adsorbing bias can not be set to be high.
On the other hand, when the polarities of the adsorbing bias and the transfer bias are the same, and when a recording sheet has high resistance, the recording sheet is charged with the positive adsorbing bias applied from the adsorbing bias applying device. As a result, the positively charged recording sheet tends to be electrostatically attracted toward the negatively charged photoreceptor. For example, when the value of the adsorbing bias is set to be relatively high, the recording sheet, which has passed a transfer position corresponding to a transfer nip part formed between the photoreceptor and the transfer bias applying device via the transfer belt, is adsorbed to the photoreceptor, instead of to the transfer belt. Thus, the recording sheet is wrapped around a part of the photoreceptor, resulting in a sheet jam. Such a sheet jam typically occurs when a thin paper having a small flexural rigidity and a basis weight of about 55 g/m2 is used as a recording sheet.
In a tandem-type image-forming apparatus, in which toner images formed at each image forming unit are sequentially transferred from the photoreceptors directly to a recording sheet, the recording sheet must be securely adsorbed to the transfer belt. By setting the polarity of the adsorbing bias applied to the recording sheet to be the same as that of the transfer bias, the electric charge on the recording sheet is discharged. In the transfer process for the first side of the recording sheet, before the adsorbing bias is applied to the recording sheet, the surface potential of the recording sheet is close to zero because the recording sheet is not charged. In this condition, when the adsorbing bias equals the transfer bias for a first color toner image, the surface potential of the recording sheet becomes nearly zero after the recording sheet passes through a transfer nip part formed between the photoreceptor and the transfer bias applying device for the first color toner image. In such a condition, the adsorbing force of the recording sheet to the transfer belt is lost. Thus, the recording sheet may not be adequately conveyed. Due to inferior sheet conveyance, deviation of the position of color toner images may occur.
According to a first aspect of the present invention, a transfer device that transfers at least one color visual image from at least one image carrier to each of first and second sides of a recording medium, includes a transfer element configured to hold and move the recording medium, at least one transfer bias applying device configured to apply a transfer bias to the recording medium by the transfer element in order to transfer the at least one color visual image from the at least one image carrier to the recording medium while the recording medium passes through at least one transfer nip part formed between the at least one image carrier and the at least one transfer bias applying device, and an adsorbing bias applying device configured to apply a first adsorbing bias to the first side and a second adsorbing bias to the second side of the recording medium to adsorb the recording medium to the transfer element. The adsorbing bias applying device is provided upstream of the at least one transfer bias applying device in a direction of movement of the recording medium. A polarity of the second adsorbing bias applied to the second side of the recording medium is opposite to a polarity of electric charge given to the recording medium due to electric discharge generated when the recording medium is separated from the at least one image carrier after passing through the at least one transfer nip part.
According to another aspect of the present invention, a method of forming an image, includes forming at least one color visual image on the at least one image carrier, applying a first adsorbing bias to a first side and a second adsorbing bias to a second side of a recording medium to adsorb the recording medium to a transfer element from an adsorbing bias applying device, applying a transfer bias to the recording medium from at least one transfer bias applying device by the transfer element, and transferring the at least one color visual image from the at least one image carrier to each of the first and second sides of the recording medium while the recording medium passes through at least one transfer nip part formed between the at least one image carrier and the at least one transfer bias applying device. In the step of applying the adsorbing bias, a polarity of the second adsorbing bias applied to the second side of the recording medium is opposite to a polarity of electric charge given to the recording medium due to electric discharge generated when the recording medium is separated from the at least one image carrier after passing through the at least one transfer nip part.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description, when read in conjunction with the accompanying drawings.